In this paper, we propose an amplifying RIS design that incorporates a single PA, enabling reflection amplification. According to our design, the impinging signal in a portion of the RIS, after getting phase configured, it is fed to the PA which feeds it to the remaining portion of the RIS that reflects it with controllable phase configuration.
The amplifying design works solely in the RF domain similar to the waveguide-based approach in [1]}, unlike full-duplex multi-antenna decode-and-forward (DF) relays, which perform down-conversion and baseband processing [2]}. Therefore, although our design looks more similar to a full-duplex multi-antenna amplify-and-forward (AF) relay, they have key differences. Relays usually perform linear processing techniques, such as maximum ratio combining (MRC), and realize power allocation optimization algorithms. Furthermore, they include bulky phase-shifter networks for transmit beamforming, and are subject to loopback self-interference [3]}. Our design is simpler and does not require complex algorithms, and prevents loopback self-interference via spatail separation in the form of back to back placement of the different RIS portions. Note that simple self-interference cancellation techniques in the RF domain can be applied [4]}. We note that a similar concept was considered in [5]}, assuming that the received signal is amplified and fed back as a leakage to the input, but without a sufficient mathematical framework and a detailed modeling of the system operation. On the other hand, we propose a unique amplifying and active RIS scheme with a solid mathematical framework, and provide the system model for it. Moreover, we analyze the theoretical bit error probability (BEP), capacity, and energy efficiency (EE) of the proposed design and compare it with a benchmark passive RIS structure. The proposed design includes interesting trade-offs. For instance, amplifying the signal further results in better system performance, but it also requires more power consumption, as well as greater maximum output power for the amplifier. Also, the advantage of the proposed scheme compared to the other schemes is that the number of active devices is not increasing with respect to the number of reflecting elements. In that way, the EE performance of the system can be boosted considerably. The contributions of this study are emphasized as follows:
